Lignin is the second most abundant organic chemical after Cellulose. It is associated with hemicellulose in both woody and fibrous plants, as the adhesive which holds the bundles of Cellulose fibrils together. It is found in the primary wall, the middle lamella and within the fibre bundle or core. FIG. 3 is a representation of a lignocellulosic fibre where the dotted area 46 is called the Middle Lamella. The Middle Lamella 46 is the glue which holds adjacent fibres together. It contains crosslinked lignin and xylan in a ratio of about 70 to 30. The Primary Wall 48 is the outer casing around the fibre core much like the casing on an underground telephone cable. It contains crosslinked lignin and xylan in about equal quantities with a small amount of cellulose to provide structural strength. The fibre bundle or core 50, 51 and 52 consists of closely bound cellulose fibrils. Each fibril is bound to the adjacent fibrils by a further coating of crosslinked lignin and xylan. The ratio of lignin to xylan in the fibre core is 30 to 70, but because of its large volume relative to the Middle Lamella and Primary Wall, 70 percent of the lignin is found in the fibre bundle. The fibrils in the fibre bundle form a slight spiral along the direction of the fibre and each fibril is hinged by an amorphous area about every 300 glucose molecules in the fibril. It is this hinge which is the weakest area in the fibril and is the point where fibrils are converted to microfibrils by the Explosion Process when operated at or above 234 degrees centigrade according to the teachings of Canadian Patent 1,217,765. Finally, the lumen 54 is a hollow area in the middle of the fibre bundle where liquids migrate through the lignocellulosic composite to provide nourishment to the plant.
The overall description of how Lignin is generated in vivo can be briefly summarized as an oxidative coupling of monolignols. Three types of starting materials are used in different proportions depending upon the plant type: coniferyl alcohol, syringyl alcohol, and to some extent, p-coumaryl alcohol. Each of these three unsaturated alcohols has four or more reactive sites, by which they may undergo coupling. As the Lignin polymer grows larger, the number of permutations for coupling increases rapidly. Furthermore, the chain becomes highly branched and cross-linking between chains occurs. Hemicellulose, in the form of a linear Xylan polymer containing side groups of 4-0-methyl-(alpha)-D-glucuronic acid residues in aspen for example, becomes cross-linked to the Lignin. These Lignin-hemicellulose bonds can be likened to spot welds which soften and become increasingly fragile above the glass transition temperature of the Xylan (165 degrees celcius). Many of the bonds between the lignol fragments are of the benzyl ether type and may be labile to electrophilic cleavage. Other bonds include the resistant carbon-carbon type and biphenyl ethers.
To conceptualize, the model of Lignin in the original lignocellulosic material is that of an infinite gel made up of a cage of Lignin which is swollen with water and has embedded in it, stiff chains of short substituted xylans (DP approx. 100-200) spot welded to the Lignin. This material acts as an adhesive to bind the macromolecular Cellulose chains together for structural purposes.